Burying a continuous elongated member in the bed of a body of water normally comprises laying the continuous elongated member along a given path on the bed of the body of water; breaking up a mass of the bed to a given depth along the path; digging a trench or generally removing the broken-up bed mass, normally at the same time the mass is broken up; and possibly covering over the continuous elongated member.
More specifically, currently used known techniques comprise removing the broken-up bed mass to form a trench in the bed of the body of water; and lowering the continuous elongated member into the trench. The removed bed mass may subsequently be used as backfill to fill the trench and bury the continuous elongated member.
Continuous elongated members are routinely buried, either completely or partly, for various reasons: to protect them from damage by blunt objects, such as anchors or nets, close to shore approaches or in relatively shallow water; and at times to avoid severe stress by natural elements, such as wave motion and current. In the case of continuous elongated members in the form of rigid pipelines laid on the bed of the body of water, the pipeline may have unsupported tracts spanning two supporting points (i.e., in which the pipeline is detached from the bed); in which case, the pipeline is particularly exposed to, and vulnerable to movement induced by, wave motion and current. At times, burying is necessary to protect pipelines from deformation (upheaval/lateral buckling) caused by thermal instability; or to protect continuous elongated members from the bed-scouring action of ice in very shallow water.
To protect the pipeline from damage, a continuous elongated member very often need simply be laid in the bottom of a suitably deep trench dug either before (pre-trenching) or, more usually, after the continuous elongated member is laid on the bed of the body of water (post-trenching). At times, the protection afforded by the trench and by natural backfilling is not enough, and the continuous elongated member must be buried using the broken-up bed mass removed from the trench, or any available bed mass alongside the trench.
Post-trenching is advantageously performed using digging machines for which the continuous elongated member laid on the bed of the body of water acts as a guide. In other words, the continuous elongated member acts as a guide and grip rail to which the drive force to move the digging machine forward is applied.
Digging machines of this type are extremely advantageous when working with continuous elongated members in the form of rigid pipelines with a constant, regular-shaped cross section, but are limited, for example, when the continuous elongated member comprises a main pipeline, and an auxiliary, so-called ‘piggyback’ pipeline, the position of which with respect to the main pipeline may change over time. For practical reasons, the auxiliary pipeline is normally strapped to the main pipeline, but the straps fail to provide for a permanently rigid, stable connection. In fact, the main and auxiliary pipelines together form an irregularly-shaped continuous elongated member, the shape of which varies over time along its length, with the result that digging machines of the type described above may affect the integrity of the auxiliary pipeline.